. American telephone practice . switch-board section serve to balance thetransmitter so that it will remain in any position. Fig. 384 shows a transmitter suspension best adapted to largemultiple boards. In this the upright post is abandoned, the hori- 534 AMERICAN TELEPHONE PRACTICE. zontal arm passing through a brass bushing in the frieze-boardabove the multiple jacks. The relays in telephone work assume an almost endless variety offorms. There has been a gradual evolution in these from the typeused in telegraphy, commonly known as the Morse relay, one ofwhich is shown in Fig. 385. For a quic
. American telephone practice . switch-board section serve to balance thetransmitter so that it will remain in any position. Fig. 384 shows a transmitter suspension best adapted to largemultiple boards. In this the upright post is abandoned, the hori- 534 AMERICAN TELEPHONE PRACTICE. zontal arm passing through a brass bushing in the frieze-boardabove the multiple jacks. The relays in telephone work assume an almost endless variety offorms. There has been a gradual evolution in these from the typeused in telegraphy, commonly known as the Morse relay, one ofwhich is shown in Fig. 385. For a quick-acting relay and a verysensitive one this type is still used in telephony where there is noparticular need to economize room. The evolution spoken of, how-ever, has been made necessary primarily on account of the necessityfor the economy of room, and also in order to produce a somewhatcheaper design than that of the old Morse relay. Of course, otherfactors enter into the problem also. In particular cases, some relays ML*J Oh. FIG. 385.—MORSE RELAY. must be of such design as to enable them to be mounted closely to-gether without producing cross-talk between them; some relaysmust be polarized so as to operate on current in one direction only;others must be differentially wound; some must be designed formaximum impedance; others for no impedance under certain condi-tions; some relays, as is the case with the old Morse, are required tomake one contact only; others are sometimes required to make threecontacts and break three; sometimes the order in which the contactsare made and broken is an important factor in the design. From allof this it will be seen that the design of a telephone relay must bemade with a view to the particular use to which that relay is to be MULTIPLE SWITCH-BOARD APPARATUS. 535 put, and that a relay adapted to one purpose in a telephone systemwill perhaps be totally unfit for use in some other portion. Taking up first the matter of supervisory relays, one
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